The invention is directed to a method for controlling an output torque of an electric variable transmission.
Continuously variable transmissions (CVT) are transmissions in which the ratio of the rotational speeds of input and output shafts of a vehicle or other machine can be varied continuously within a given range.
An electric variable transmission (EVT) allows for CVT action by using more than one power input to produce a single output, and is a popular method for transmitting power in hybrid vehicles, enabling an Internal Combustion Engine (ICE) to be used in conjunction with motor/generators for vehicle propulsion, and having the ability to control the portion of the mechanical power used directly for propelling the vehicle and the portion of mechanical power that is converted to electric power and recombined to drive the vehicle. At least one of the power inputs is an electric motor.
An EVT is usually designed with a differential gear system performing a “power-split” function, by which a portion of the mechanical power is connected directly through the transmission and another portion is split off for subsequent conversion to electrical power via a motor/generator. In a differential gear with three input/output shafts the rotational speed and the torque of each shaft is determined by the rotational speed and torque of both of the other shafts. For instance in a car when one wheel is running free (e.g., when the car is jacked up) the other wheel of the same axis connected to the same differential gear will not produce traction when the engine is running. Also when both wheels are jacked up and the input shaft kept from rotating by stopping the engine, rotating one wheel will cause the other one to rotate in the opposite direction with the same revolution speed. When a differential gear is used in an EVT the revolution speed of the transmission output can be controlled by direction and ratio of the rotations of all input shafts. This means the car equipped with the EVT may be accelerated by accelerating the combustion engine or the electric motor while the respective other propulsion unit is kept at the same revolution speed.
The EVT and power sources are controlled to provide a balance between the power sources that increases vehicle fuel economy while providing advantageous performance when needed. The EVT may also be used to provide electrically generated power to charge large storage batteries for subsequent electric motor propulsion as needed, or to convert vehicle kinetic energy to electricity through ‘regenerative braking’ during deceleration. Various configurations of power generation, usage and balance can be implemented with a EVT, enabling great flexibility in propelling hybrid vehicles.
The maximum and minimum transmission output power of the EVT depends on the variation of energy of the inertias in the power train. In dynamic state the transmission power output may therefore fluctuate heavily, or may be subject to discontinuities, since part of the energy is required for changing the revolution speed against the inertia effects, which are significant due to the relatively high masses involved. These discontinuities may appear inconvenient to the car's driver and passengers.
German patent document DE 10 2005 006 369 discloses a method in which minimum and maximum battery powers are given in data sets correlated to battery conditions such as charging state, temperature, battery voltage and battery usage.
German patent document DE 10 2004 043 589 discloses a method for determining a traction power distribution in a hybrid vehicle, wherein a potential battery power is determined from the actual charging state, the set charging state, characteristic charging power limits and discharging power limits of the battery depending of the driving style (the so called sportivity index), a value for a minimum and a maximum charging state and the minimum and maximum power of the combustion engine.
U.S. Pat. No. 6,946,818 discloses a method of providing closed-loop control of power flowing into and out of an electrical energy storage system, including the steps of: determining a charge power limit comprising a maximum charge power during each of a plurality of control loops; determining a discharge power limit comprising a maximum discharge power during each of the plurality of control loops; comparing the charge power limit and the discharge power limit during each of the plurality of control loops; and providing a charge power limit output and a discharge power limit output for use in a subsequent control loop, which are based upon the charge power limit and the discharge power limit. The charge power limit output and discharge power limit output are equal to the discharge power limit and charge power limit, respectively, when the discharge power limit is greater than the charge power limit; and the charge power limit output and discharge power limit out-put are selected from the group consisting of the charge power limit, the discharge power limit and zero, when the discharge power limit is less than or equal to the charge power limit.
U.S. Pat. No. 6,868,318 discloses a method for improving the performance of an energy storage system that incorporates a high density electrical energy storage device, such as a battery or ultracapacitor. The method may be implemented in an energy storage system of a hybrid electric vehicle (HEV) as a computer control algorithm for controlling the discharge power limits of an energy storage device, such as a battery. The method allows the discharge power limits of the battery to be temporarily expanded under vehicle launch conditions where the power demands are high, thereby making additional stored energy available for use under such conditions by improving battery utilization and providing more consistent vehicle launch characteristics than would otherwise be available.
Japanese patent document JP 2002291104 discloses a vehicle battery controller in which the limit values of residual capacity and temperature are stipulated to control the charging and discharging of the battery 7. The charging and discharging are controlled by the controller 3 in such a way that the residual capacity and temperature do not exceed the limit values. Also, a pedal angle detector 2 is provided to detect the angle of an accelerator pedal 1. Based on the detected results of the amount and change speed of the angle of the accelerator pedal, the controller 3 determines whether or not acceleration is operated. When accelerated, the limit is relaxed for discharging by the residual amount and temperature.
European patent document EP 0 868 005 discloses an arrangement for powering a driving shaft. The difference between the output power of a combustion engine and a target power is acquired by an electric motor with a pair of rotors that may rotate independently, and thus determine the power drain or power output of the electric motor. The arrangement comprises first and second correction devices for correcting the target power of the engine. When the battery is charged or discharged with electric power out of a predetermined range the target power of the drive shaft is corrected.
One object of the present invention is to provide an improved method for controlling the output torque of an electric variable transmission.
This and other objects and advantages are achieved according to the invention, which provides a method for controlling an output torque of an electric variable transmission with a combustion engine and at least one electric motor. The combustion engine and the electric motor or motors may propel the output. The electric motors may also be run in a generator mode, thereby recuperating or regenerating a battery. Furthermore one of the electric motors may be in driving mode and at least partly provided with energy by the other motor, which is operating in a generator mode at the same time. In the electric variable transmission a revolution speed of the combustion engine may be changed by the combustion engine's torque as well as by the electric motor's torque.
The method according to the invention comprises the steps of:                restricting the battery power to a maximum continuous battery power when the transmission is in a stationary operating point with a continuous revolution speed;        raising the battery power and restricting it to a maximum transient battery power (greater than the maximum continuous battery power) in a non-stationary operating point, upon a request for changing the gear-ratio (ratio between transmission input and output-speed), or upon a request for changing the transmissions output revolution speed.        
The method allows for a steadier and smoother characteristic of the transmission output torsion, thus providing a convenient driving impression. The method may also be applied to parallel hybrids, where the electric motor is used to support the synchronization of revolution speeds when changing gears. In this context, shorter cycle times may be achieved by a short-term increase of the battery power, depending on a requested revolution speed gradient.
The battery discharging power may be restricted to a maximum transient value when boosting in the non-stationary operating point, while the battery charging power may be restricted to a maximum transient value when recuperating in the non-stationary operating point.
The absolute values of the maximum transient battery discharging power and the maximum transient charging power may be different from each other or equal each other, as may be the maximum continuous battery discharging power and the maximum continuous battery charging power. Usually these limits are specified by the battery's manufacturer.
In a preferred embodiment of the invention the battery power required for boosting the electric motor, or the battery power that needs to be absorbed by the battery when recuperating, is calculated as the sum of the mechanical powers of the electric motors and an overall loss of power due to friction, heat, ohmic drop, battery losses and the like.
In the stationary operating point, the battery power required may be calculated by summing the transmission output power and the overall loss of power, and subtracting the power of the combustion engine.
In the non-stationary operating point the required battery power may be calculated by summing the transmission output power, the overall loss of power, and an inertia power required for compensating inertia operative in the transmission, and subtracting the power of the combustion engine. The inertia power may be known from a characteristic curve or map, depending on parameters such as input transmission revolution speed or a derivative thereof.
In a preferred embodiment the battery power required in the non-stationary operating point is calculated by summing the transmission output power, the overall loss of power, and a function of an inertia power required for compensating inertia operative in the transmission, and subtracting the power of the combustion engine.
In another preferred embodiment, the method may be applied to an electric variable transmission coupled to two electric motors (i.e., a so called split power transmission). It may also be applied in a hybrid vehicle.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.